Powder coating resin process

ABSTRACT

A curable powder resin composition comprising an aminoplast and a polyester or polyacrylic resin. The aminoplast is a condensate of a methylolurea or methylolaminotriazine with an aliphatic alcohol and a reactive compound containing at least one reactive group selected from the group consisting of hydroxyl and amide.

This application is a continuation in part of application Ser. No.317,901, filed Dec. 26, 1972 abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is in the field of curable powder coating resincompositions comprising a blend of aminoplast crosslinking agents andpolymeric materials containing active hydrogen groups, suitable forpowder spray applications.

2. Description of the Prior Art

Powder coatings are prepared from powdered resins applied byelectrostatic or fluidized bed processes. In the former, the powderedresin is applied by electrostatic spray gun to a substrate of oppositecharge which is then baked to fuse the powder particles into a smoothcoating. In the latter process the preheated substrate is immersed in afluidized mass of powdered resin so that the powder adheres to thesubstrate which is then baked to fuse the powder into a smooth coating.Powder coatings possess significant advantages over conventionallyapplied solvent finishes in economy, ease of application, single coatpotential and absence of sagging. Moreover, air pollution is eliminated.

Thermoplastic and thermosetting resins may be applied by powder coatingtechniques. Thermosetting powders offer several advantages; namely, theyrequire no primer, they yield tough flexible films of superior chemicaland solvent resistance, they provide adequate protection to thesubstrate in thinner films, they accept higher pigmentation levels andthey may be applied at lower curing temperatures.

Powdered epoxy resins have been successfully developed for powdercoatings. Unfortunately, epoxy coatings weather poorly, rapidly losinggloss. Hence, for exterior uses, thermosetting acrylic or polyesterpowder coatings would be preferred. Attempts to produce such coatingshave been based on systems which contain aminoplast crosslinking agentsand acrylic or polyester resins with reactive hydrogen groups such ashydroxyl, carboxyl, or amide groups. To date, such systems containingconventional aminoplasts; for example, methylol and alkoxymethylderivatives of ureas, melamines and benzoguanamines, have beenunsatisfactory because of poor package stability. The poor packagestability is associated with the plasticizing and cold flow effects ofthe conventional low molecular weight aminoplasts, causing clumping andsintering or fusing of the powdered resin. Clumping and sintering areundesirable since they impair fluidization of the powder duringapplication to the substrate, they cause segregation of the powder intovarying degrees of density, and they contribute to non-uniform coatingswith decreased gloss. The cold flow may be avoided by cutting back onthe quantity of aminoplast but with sacrifice in physical properties andsolvent resistance of the cured film, or by the use of high molecularweight acrylic or polyester resins, an expedient which impairsfusibility of the powder and flow so that the final coating lacks glossand coherence, or by the use of acrylic or polyester resins of highglass transition temperature so that the glass transition temperature ofthe powdered composition is increased but the cured film is embrittledand loses impact strength.

Thus, there is a need for powdered resin compositions comprising a blendof aminoplast crosslinking agents and acrylic or polyester materialscontaining active hydrogen groups, which have adequate package stabilityand flowability and which yield powder coatings which when cured haveadequate physical properties and solvent resistance.

SUMMARY OF THE INVENTION

The curable powder resin compositions of this invention comprise a blendof aminoplast crosslinking agent with glass transition temperature atleast -10°C. and a reactive polymeric material containing at least oneclass of reactive groups selected from the group consisting of alcoholichydroxyl groups and amide groups wherein the amount of the reactivegroups is at least 0.13 percent by weight and not more than 18 percentby weight of the reactive polymeric material. The powder resin blend hassufficient aminoplast crosslinking agent to give a cured coating withadequate physical properties and solvent resistance. The weight ratio ofaminoplast crosslinking agent to reactive polymeric material is in therange of 15:85 to 30:70. The aminoplast crosslinking agent, however,differs from conventional aminoplasts by having a higher glasstransition temperature allowing the blend to have a sinteringtemperature above 40°C. and, hence, a better package stability thanblends containing conventional aminoplasts.

The aminoplast crosslinking agents comprise a condensate of amethylolamino compound such as a methylolurea or methylolaminotriazinewith a first compound which is a C₁ to C₄ aliphatic alcohol and a secondcompound which contains at least one reactive group selected from thegroup consisting of hydroxyl or amide. The molar ratio of the aliphaticalcohol to the methylolamino compound is at least 2. The molar ratio ofthe methylolamino compound to the second compound is between about 1:0.5and about 1:2 when the second compound contains one reactive group permolecule and is between about 1:0.5 and about 1:1.5 when the secondcompound contains two or more reactive groups per molecule. Themethylolamino compound contains no more than one unmethylolated N-H bondper molecule. At least 50 percent of the methylol groups of themethylolamino compound are condensed with the first and secondcompounds. The glass transition temperature of the condensed aminoplastcrosslinking agent is between -10° and 100°C.

The reactive polymeric material which is the second component of thecurable powder resin composition has a glass transition temperature inthe range of 60° to 100°C. and a molecular weight less than 25,000,preferably, between 500 and 10,000.

The curable powder resin compositions are applied to substrates byelectrostatic spray coating. The coated substrate is baked at atemperature between 120° and 220°C. for from 10 to 60 minutes. The curedsurface coating exhibits good solvent resistance and toughness.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The first component of the curable powder resin composition of thepresent invention, the aminoplast crosslinking agent, is the alkylatedreaction product of a methylolated urea or an aminotriazine with acoreactant containing at least one class of reactive groups selectedfrom the group consisting of hydroxyl groups and amide groups.

The methylolated ureas and aminotriazines are prepared by conventionalalkaline addition reaction of the urea or aminotriazine withformaldehyde. The preferred reactants are urea, melamine andbenzoguanamine. Methylolation is carried out so that on average no morethan one N-H bond per molecule of urea or aminotriazine remainsunreacted. Preferably, substantially complete methylolation is carriedout so that the preferred reactants yield hexamethylolmelamine andtetramethylol-benzoguanamine, respectively.

The methylol derivatives are then condensed under conventional acidconditions with a co-reactant of number average molecular weight up to500 containing at least one class of reactive groups selected from thegroup consisting of hydroxyl groups and amide groups. Preferred classesof co-reactants are: Aliphatic diols and polyols such as ethyleneglycol, 1,2-propanediol, propylene glycol, neopentyl glycol,1,4-bis(hydroxymethyl) cyclohexane, hydrogenated bisphenol-A, glycerol,sucrose; aliphatic diamides and polyamides such as succinamide andadipamide; aromatic alcohols such as benzyl alcohol, oligomericpoly(styrene-co-allyl alcohol); aromatic amides such as benzamide,o-toluamide, m-toluamide, p-toluamide, terephthalamide; aromaticsulfonamides such as benzenesulfonamide, o-toluenesulfonamide,m-toluenesulfonamide, p-toluenesulfonamide and terephthalsulfonamide;and phenols such as phenol, o-cresol, m-cresol, p-cresol, and α- andβ-naphthol.

In the reaction with monofunctional co-reactant such as benzyl alcohol,the molecular ratio of methylolated urea or methylolated aminotriazineto monofunctional co-reactant is in the range of 2:1 to 1:2, preferablybetween about 1:1 and about 1:2. With polyfunctional co-reactant such asethylene glycol and terephthalamide, the molecular ratio of methylolatedurea or methylolated aminotriazine to polyfunctional co-reactant isbetween 2:1 and 2:3, preferably between about 2:1 and about 1:1.

The condensation is carried out under conventional acid conditions inthe presence of a C₁ to C₄ aliphatic alcohol which also condenses withthe methylolamino compound so that the reaction product containsalkoxymethyl groups, also. Alternatively, the condensation may becarried out in separate steps in any order and it is often convenient tostart with commercially available C₁ to C₄ alkylated methylolated ureasor aminotriazines which may contain up to 50 weight percent of dimersand trimers and condense them with the monofunctional or polyfunctionalco-reactants to obtain the desired condensates.

Condensation of the aminomethylol compound may be carried out so thatall the methylol groups are reacted. At least half the methylol groupsare reacted so that the curable powder resin composition will haveadequate shelf-stability without premature cure. It is preferred to haveat least two C₁ to C₄ alkoxy groups per molecule of aminomethylolcompound to obtain a balanced reactivity in the aminoplast crosslinkingagent. The aliphatic alcohol and reactive compound are selected so thatwhen they are reacted with the methylolamino compound in the molarratios set forth above, the condensate has a glass transitiontemperature between -10° and 100°C. and more preferably between 10° and80°C. so that when the condensate is formulated with the polymericcomponent of the curable powder resin composition of the presentinvention, the blend yields a powder with a sintering or fusiontemperature in the range of 40° to 100°C.

The second component of the curable powder resin compositions of thepresent invention is a reactive polymer containing at least one class ofreactive groups selected from the group consisting of alcoholic hydroxylgroups, carboxyl groups and amide groups and is preferably a polyesterresin or an acrylic resin for exterior coating applications.

When polyester resins are used they are prepared by reacting apolycarboxylic acid including dicarboxylic acids with a polyhydricalcohol, including the glycols. The polycarboxylic acids may be eithersaturated, i.e., free of non-benzenoid unsaturation, or they may beethylenically unsaturated.

If the polyester resins used contain preponderant amounts of thepolycarboxylic acid, such polyester should be reacted with dihydricalcohol until an acid number of between about 1 to 25 is reached and,preferably, an acid number of between about 3 to 10. If the polyesterresin is prepared by using a preponderance of alcoholic hydroxyl groups,the ingredients forming the polyester are permitted to react until ahydroxyl number of not less than about 5 is reached and, preferably,between about 11 and 200. The polyester resins are prepared by thegeneral technique employed in the preparation of polyester resins. Theycan be conveniently made by stirring the reactants together and heatingthem while stirring, to a temperature of 240°C., while an inert gas(e.g., nitrogen) is continuously passed through the reaction mixture toremove the low boiling products formed during the esterificationreaction. At the end of the esterification reaction, vacuum is appliedof about 100 mm. Hg and the ester is polymerized and glycol is distilledoff. The polyester resin has a glass transition temperature between 60°and 100°C. and for adequate flowability of the powder resin composition,the polyester resin component has a molecular weight of less than 25,000and preferably between 500 and 10,000.

When acrylic resins are used, they are prepared by copolymerizingmonomers such as methyl acrylate, ethyl, acrylate, propyl acrylate,butyl acrylate, octyl acrylate, decyl acrylate, lauryl acrylate, methylmethacrylate, ethyl methacrylate, butyl methacrylate, heptylmethacrylate, decyl methacrylate, methyl ethacrylate and the like withacidic co-monomers such as acrylic acid, methacrylic acid, crotonicacid, maleic acid, fumaric acid, itaconic acid and the like, or withalcoholic hydroxy comonomers such as the hydroxyalkyl esters ofα,β-unsaturated mono- and di-basic acids; for example 2-hydroxyethylacrylate, 2-hydroxypropyl acrylate, 3-hydroxypropyl acrylate,2-hydroxybutyl acrylate, 8-hydroxyoctyl acrylate, 2-hydroxyethylmethacrylate, 5-hydroxyhexyl methacrylate, 10-hydroxydecyl methacrylate,2-hydroxyethyl crotonate, bis-(hydroxyethyl) maleate, and the like, orwith amide comonomers such as acrylamide, methacrylamide, ethacrylamide,and the like. Additionally, one can make use of otherhydroxyl-containing polymerizable monomers such as methylolacrylamide,methylolmethacrylamide and the like. The monomers and ratio of monomersis selected so that the acrylic resin has at least 0.13 percent byweight of alcoholic hydroxyl groups, carboxyl groups and amide groupsand not more than 18 percent by weight based on the total weight ofacrylic resin, so that the resin has on average at least two reactivehydroxyl, carboxyl or amide groups per molecule, and so that the acrylicresin has a glass transition temperature in the range of 60° to 100°C.The glass transition temperature is a function of the composition of theacrylic resin and its molecular weight. For adequate flowability of thepowder resin composition, the molecular weight of the acrylic resinshould not be more than 25,000 and preferably it should be between 500and 10,000.

Still further, one can use such polymerizable compounds in the acrylicresin as styrene, alkyl styrenes such as o-, m- or p-methylstyrene,halostyrenes such as o-, m-, or p-chlorostyrene, acrylonitrile,methacrylonitrile, vinyl chloride, vinylidene chloride and vinyl esterssuch as vinyl acetate, vinyl propionate or vinyl octoate provided nomore than 50 percent by weight of the acrylic resin comprises suchmonomers and provided that the aforementioned criteria of glasstransition temperature, content of reactive groups and molecular weightare met.

The acrylic resins are conveniently made by conventional additionpolymerization techniques with free-radical initiators and chaintransfer agents to control the molecular weight, in bulk, solution oremulsion systems with recovery of the resin by conventional removal ofunreacted monomer, solvent or water.

The weight ratio of aminoplast crosslinking agent to polyester oracrylic resin is in the range of 15:85 to 30:70. Below this range, thereis insufficient aminoplast crosslinking agent to provide the degree ofcrosslinking necessary for adequate physical properties in the curedcoating. Above this range, the aminoplast crosslinking agent depressesthe sintering temperature of the powder resin composition below 40°C.and impairs the package stability and storability of the powder causingclumping, imperfect fluidization of the powder during application to thesubstrate, segregation of the powder into varying degrees of density,and decreased gloss in the final coating.

The invention is illustrated by the following examples in which unlessotherwise specified, all parts are parts by weight.

Preparation of Aminoplast Crosslinking Agents

Examples 1 to 9 illustrate the preparation of the aminoplastcrosslinking agents which are the first component of the curable powderresin composition of the present invention.

EXAMPLE 1

In a suitable reactor, 320 parts by weight of methylatedmethylolmelamine with a degree of methylolation (i.e. average number ofmethylol groups per melamine molecule) of 5.5 and a degree ofmethylation (i.e. average number of methyl groups per molecule ofmelamine) of 3.0 and a number average molecular weight of approximately500 are heated to 120°C. and 240 parts of hydrogenated bisphenol arestirred in. When a clear solution forms, 0.2 parts of citric acid areadded. The temperature is held at 120°C. and the reaction mixture isdistilled until 44 parts have been collected. The residue is dischargedinto a cold tray. The product has a glass transition temperature ofapproximately 15°C. measured on a duPont 900 Differential ThermalAnalyzer with a heating rate of 20°C. per minute.

EXAMPLE 2

The procedure of Example 1 is repeated with 62 parts of ethylene glycolin place of the hydrogenated bisphenol-A. The resin product has a glasstransition temperature of approximately -7°C.

EXAMPLE 3

The procedure of Example 1 is repeated with 320 parts styrene-allylalcohol copolymer of hydroxyl number 57 in place of the hydrogenatedbisphenol-A. The reaction mixture is distilled until 20 parts have beencollected. The glass transition temperature of the resin product is27°C.

EXAMPLE 4

The procedure of Example 1 is repeated with 400 parts of a methylatedmethylolmelamine with a degree of methylolation of 5.8, a degree ofmethylation of 5.5, and a number average molecular weight ofapproximately 400. The glass transition temperature of the resin productis approximately 10°C.

EXAMPLE 5

The procedure of Example 1 is repeated with 475 parts of a butylatedmethylolbenzoguanamine with a degree of methylolation of 4.0 and adegree of butylation of 2.7 and 240 parts of hydrogenated bisphenol-A.The glass transition temperature of the resin product is 27° to 30°C.

EXAMPLE 6

The procedure of Example 1 is repeated with 137 parts ofp-toluenesulfonamide in place of hydrogenated bisphenol-A. The glasstransition temperature of the resin product is approximately 15°C.

EXAMPLE 7

The procedure of Example 1 is repeated with 164 parts terephthalamide inplace of hydrogenated bisphenol-A. The glass transition temperature ofthe resin product is 30°-35°C.

EXAMPLE 8

The procedure of Example 1 is repeated with 121 parts benzamide in placeof hydrogenated bisphenol-A. The glass transition temperature is 0°-5°C.

EXAMPLE 9

The procedure of Example 1 is repeated with 300 parts of a methylatedmethylolmelamine with a degree of methylolation of 5.5 and a degree ofmethylation of 3.0 and 200 parts of p-toluenesulfonamide. The glasstransition temperature of the resin product is 29°C.

Preparation and Evaluation of Curable Powder Resin Compositions

In preparing curable powder resin compositions according to the presentinvention, the reactive polymer and the aminoplast are convenientlymixed at a temperature just above the melting point of the highermelting component in an extruder. The mixture is cooled to solidify itand subsequently crushed and ground in a suitable grinding device suchas a pin disc mill to a free-flowing powder having a particle size rangeof approximately 20-150 microns. The free-flowing powder so obtained canbe subjected to a screening treatment to adjust the desired averageparticle size for the specific type of application to be employed. Fortest purposes, powders of average particle size of 80 microns are used.

The homogeneous powder resin and compositions may also include variouswell-known functional modifiers such as grinding aids, curing agents oraccelerators, flow control agents, surface active agents, heat-stableorganic or inorganic pigments, inert fillers, inhibitors, abrasives andplasticizers in their usual effective proportions.

The acid number of the powder resin composition is in the range of 1 to18 and preferably in the range of 3 to 10 so that sufficient acid ispresent to catalyze the interaction of the components of the compositionat elevated temperatures but not enough to impair stability at storagetemperatures. If there is insufficient acid present in the resincomponents to provide the required acidity, conventional acid catalystsmay be added.

Also, small amounts of other resins or other organic compounds toimprove the film-forming properties may be incorporated. It will beunderstood that the amount of these substances must be such that thephysical properties of the powders and cured coating are not impaired.

After application of the coating compositions to the substrate, thecoatings are cured at a temperature from 120° to 250°C. for a period of10 to 60 minutes. The curing treatment is preferably carried out for 20to 40 minutes at 150° to 200°C. The thickness of the cured film may bevaried in the range of 0.5 mil to 20 mil. For test purposes, films of1.5 mil thickness are used.

In evaluation of the curable powder resin compositions of the presentinvention, compatibility of the aminoplast and the reactive polymer, thesintering temperature of the powder and the properties of the bakedcoating are determined. Compatibility of the aminoplast crosslinkingagent and the reactive polymer is determined by coating a solution ofaminoplast and polymer onto a glass plate. When the solvent hasevaporated, the coated film is examined and if it is clear, theaminoplast and polymer are judged to be compatible. All the aminoplastsof Examples 1- 9 are found to be compatible with polyester resin A ofExample 10 and the acrylic resin of Examples 14 and 15.

The sintering temperature is determined from a standard glass transitiontemperature curve obtained on a duPont 900 Differential Thermal Analyzerwith a heating rate of 20°C. per minute. The lower of the two tangentialpoints of the glass transition temperature is found to correspond to thesintering temperature of the powder since the powder remainsfree-flowing and unsintered when it is maintained at 3° below thistemperature for 2 weeks but sinters within 2 days at this temperature.

Testing of Coated Sheets

The baked cured panels are aged overnight at 77±2°F. and 50 percentrelative humidity. The coatings are evaluated for appearance includingpinholes, craters and abnormal roughness. They are tested by thefollowing procedures:

a. Impact Test

The reverse impact test involves a two-pound rod with a nominal diameterof 1 inch and a length of 18 inches having a hemispherical tip of 1/2inch radius. The rod is dropped vertically a certain number of inches onthe uncoated side of the panel and the ability of the panel to deform ismeasured versus the ability of the coating to withstand this deformationwithout cracking. The panel on impact deforms into a cup deformation.The rounded cup is tested by placing a pressure-sensitive cellophanetape of 0.5 inch in width over the raised cup. The cellophane tape isstripped to determine whether or not the coating has lost its adhesionduring the deformation. The rod is dropped from various heights and thetest has a readout of inch-pounds versus adhesion of the coating.

b. Rub Test

A pool of solvent is placed on the coated panel. The coating is rubbedgently with a cheesecloth covered finger. The number of rubs before thecoating begins to be removed is noted.

c. Pencil Hardness Test

The coated side of the panel is placed face-up on a flat surface. Theoperator presses on the panel with pencils of varying hardness. Thepanel fails when the lead penetrates the coating and the hardness ratingis determined by that hardness which does not penetrate. The range ofhardness for pencils ranges from 6B, being the softest, to 9H, being thehardest, i.e., 6B to B, HB, F, H to 9H. The combination coatings taughtvary in hardness from F to 6H. Hardness of 2H or better is preferred.

EXAMPLE 10

A powder resin composition is prepared from the following ingredientsblended at 100°C. for 3 minutes in an extruder.

    ______________________________________                                                           Parts by Weight                                            ______________________________________                                        Polyester Resin A    250                                                      Aminoplast Product of Ex. 5                                                                        108                                                      Titanium Dioxide     250                                                      MODAFLOW RESIN MODIFIER                                                                            0.8                                                      (product of Monsanto Company)                                                 ______________________________________                                    

The polyester Resin A is the reaction product of isophthalic acidneopentyl glycol and trimethylolethane with a 20 percent excessequivalent hydroxyl, an acid number of 6.9 and a glass transitiontemperature of 72°C. and a number average molecular weight of 2,000.

The sintering temperature of the powder resin composition is 42°C. Theweight ratio of aminoplast to polyester is 30:70.

Mild steel panels are coated with the powder resin composition by theelectrostatic spray-coating method and baked at 175°C. for 30 minutes.The baked coating has a forward impact of 45 inch-pounds, a reverseimpact of 23 inch-pounds and a methyl ethyl ketone rub value of morethan 200.

EXAMPLE 11

A powder resin composition is prepared from the following ingredientsblended at 100°C. for 3 minutes in an extruder.

    ______________________________________                                                           Parts by Weight                                            ______________________________________                                        Polyester Resin A    250                                                      Aminoplast Product of Ex. 2                                                                        63                                                       MODAFLOW RESIN MODIFIER                                                                            0.8                                                      ______________________________________                                    

The sintering temperature is 40°C. The weight ratio of aminoplast topolyester is 20:80.

EXAMPLE 12

    ______________________________________                                                           Parts by Weight                                            Polyester Resin A    250                                                      Hexamethoxymethylmelamine                                                                          63                                                       MODAFLOW RESIN MODIFIER                                                                            0.8                                                      ______________________________________                                    

The weight ratio of aminoplast to polyester is 20:80. The sinteringtemperature is below room temperature, so that it is impossible to grindor spray the composition under normal conditions.

EXAMPLE 13

A powder resin composition is prepared from the following ingredientsblended at 100°C. for 3 minutes in an extruder.

    ______________________________________                                                           Parts by Weight                                            ______________________________________                                        Polyester Resin A    250                                                      Hexamethoxymethylmelamine                                                                          29                                                       Titanium Dioxide     250                                                      MODAFLOW RESIN MODIFIER                                                                            0.8                                                      ______________________________________                                    

The weight ratio of aminoplast to polyester is 10:90. The sinteringtemperature of the powder resin compound is approximately 25°C.

Mild steel panels were coated with the powder resin composition by theelectrostatic spray-coating method and baked at 175°C. for 30 minutes.The baked coating has a forward impact of 20 inch-pounds, a reverseimpact of less than 10 inch-pounds, and an MEK rub value of 50.

A comparison of Examples 10-13 illustrates that powder resincompositions containing the aminoplast crosslinking agents of thepresent invention have adequate sintering temperatures for packagestability and yield coatings sufficiently cured for adequate impactstrength and solvent resistance. In contrast, compositions containinghexamethoxymethylmelamine have poor package stability and as in Example13 when the amount of hexamethoxymethylmelamine is reduced to raise thesintering temperature, a drastic loss in impact strength and solventresistance is observed.

EXAMPLE 14

A powder resin composition is prepared from the following ingredientsblended in a Banbury mixer at 100°C. for 15 minutes.

    ______________________________________                                                           Parts by Weight                                            ______________________________________                                        Acrylic Resin B      250                                                      Aminoplast Product of Ex. 9                                                                        63                                                       Titanium Dioxide     250                                                      MODAFLOW RESIN MODIFIER                                                                            0.8                                                      ______________________________________                                    

The weight ratio of aminoplast to acrylic resin is 20:80. The sinteringtemperature of the resin composition is approximately 54°C.

The acrylic resin B is an addition copolymer of 14.7 parts by weight ofstyrene, 46 parts by weight of methyl methacrylate, 10.4 parts by weightof 2-hydroxyethyl methacrylate and 28.9 parts by weight of butylmethacrylate. The glass transition temperature is approximately 58°C.

Mild steel panels are coated with the powder resin by means ofelectrostatic spray-coating and baked at 175°C. for 30 minutes. Thebaked panel has a forward impact resistance of 4 inch-pounds and an MEKrub value of 200.

EXAMPLE 15

A powder resin composition is prepared from the following ingredientsblended in a Banbury mixer at 100°C. for 15 minutes.

    ______________________________________                                                           Parts by Weight                                            ______________________________________                                        Acrylic Resin C      250                                                      Aminoplast Product of Ex. 9                                                                        63                                                       Titanium Dioxide     250                                                      MODAFLOW RESIN MODIFIER                                                                            0.8                                                      ______________________________________                                    

The weight ratio of aminoplast to acrylic is 20:80. The acrylic Resin Cis an addition copolymer of 19.5 parts by weight of styrene, 46 parts byweight of methyl methacrylate, 5.6 parts by weight of acrylamide and28.9 parts by weight of butyl methacrylate. The glass transitiontemperature is >60°C.

The sintering temperature of the powder resin composition is >55°C.

Mild steel panels are coated with the powder resin by means ofelectrostatic spray coating and baked at 175°C. for 30 minutes. Thebaked panel has a forward impact resistance of 4 inch-pounds and an MEKrub value of 200.

What is claimed is:
 1. A curable powder resin composition with asintering temperature above 40°C. and curable in the temperature rangeof 120° to 220°C., comprising:A. from about 15 to 30 parts by weight ofa condensate of a methylolamino compound, an aliphatic alcoholcontaining from 1 to 4 carbon atoms and an aromatic sulfonamide ofnumber average molecular weight less than 500; wherein the methylolaminocompound is a methylolurea or methylolaminotriazine containing no morethan one unmethylolated N-H bond per molecule, wherein the molar ratioof aliphatic alcohol to methylolamino compound is at least 2, whereinthe molar ratio of the methylolamino compound to the aromaticsulfonamide is from about 1:0.5 to about 1:2 when the reactive compoundcontains one reactive group per molecule and is from about 1:0.5 toabout 1:1.5 when the aromatic sulfonamide contains 2 or more reactivegroups per molecule, wherein at least about 50 percent of the methylolgroups of the methylolamino compound have been condensed with thealiphatic alcohol or the aromatic sulfonamide and wherein the glasstransition temperature of the condensate is in the range of from -10° to100°C.; and B. from about 85 to 70 parts by weight of a polyester resinor an acrylic resin wherein the resin has a glass transition temperaturein the range of from 60° to 100°C., and a number average molecularweight in the range of 500 to 25,000, and contains alcoholic hydroxylgroups, carboxyl groups or amide groups, heat reactive with thecondensate of the methylolamino compound, wherein the polyester resinhas an acid number in the range of 1 to 25 or a hydroxyl number in therange of 11 to 200, and wherein the acrylic resin contains from 0.13 to18 percent by weight of hydroxyl, carboxyl, or amide groups.
 2. Thecurable powder resin composition of claim 1 wherein the methylolaminocompound is a methylolmelamine, a methylolurea or amethylolbenzoguanamine, and wherein the aromatic sulfonamide is selectedfrom the group consisting of benzenesulfonamide, o-, m-, andp-toluenesulfonamide, and terephthalsulfonamide.
 3. The curable powderresin composition of claim 1 wherein the methylolamino compound is amethylolmelamine, the aliphatic alcohol is methanol and the reactivecompound is selected from the group consisting of o-, m-, andp-toluenesulfonamide.
 4. The curable powder resin composition of claim 1wherein the sintering temperature is in the range of 40°-100°C., whereinthe glass transition temperature of the condensate is in the range 10°to 80°C., and wherein the molecular weight of the polyester resin or theacrylic resin is in the range of 500 to 10,000.
 5. The curable powderresin composition of claim 1 wherein the molar ratio of themethylolamino compound to the reactive compound is between about 1:1 andabout 1:2 when the reactive compound contains one reactive group permolecule and is between about 1:0.5 and about 1:1 when the reactivecompound contains 2 or more reactive groups per molecule.
 6. A curablepowder resin composition with a sintering temperature in the range of40° to 100°C., and curable in the temperature range of 120° to 220°C.,comprising:A. from about 15 to 30 parts by weight of a condensate of amethylolmelamine, an aliphatic alcohol containing from 1 to 4 carbonatoms and toluenesulfonamide, wherein the methylolmelamine contains nomore than one unmethylolated N-H bond per molecule, wherein the moleratio of the aliphatic alcohol to the methylolmelamine is at least 2,wherein the molar ratio of the methylolmelamine to thetoluenesulfonamide is from about 1:0.5 to about 1:2, wherein at leastabout 50 percent of the methylol groups of the methylolmelamine havebeen condensed with the aliphatic alcohol or the toluenesulfonamide andwherein the glass transition temperature of the condensate is in therange of from -10° to 100°C; and B. from about 85 to 70 parts by weightof a polyester resin or an acrylic resin wherein the resin has a glasstransition temperature in the range of 500 to 25,000, and containsalcoholic hydroxyl groups, carboxyl groups or amide groups, heatreactive with the condensate of the methylolamino compound, wherein thepolyester resin has an acid number in the range of 1 to 25 or a hydroxylnumber in the range of 11 to 200, and wherein the acrylic resin containsfrom 0.13 to 18 percent by weight of hydroxyl, carboxyl, or amidegroups.